1. Field of the Invention
The present invention relates to a cellulose acylate film, a stretched cellulose acylate film, and a method and an apparatus for producing the same, particularly to a technique for producing a cellulose acylate film before stretching for a stretched cellulose acylate film having qualities suitable for liquid crystal display devices by melt film forming.
2. Description of the Related Art
Conventionally, it has been attempted to enlarge viewing angles by stretching a cellulose acylate film for exhibiting in-plane retardation (Re) and retardation (Rth) in the thickness direction and using the film as a retardation film in liquid crystal display devices.
Methods of stretching such a cellulose acylate film include a method of stretching a film longitudinally (in the lengthwise direction)(longitudinal stretching), a method of stretching a film transversely (in the width direction)(transverse stretching), and a method of performing longitudinal stretching and transverse stretching simultaneously (simultaneous stretching). Of these, longitudinal stretching has been often employed because of the compactness of the equipment. In longitudinal stretching, a film is generally heated to its glass transition temperature (Tg) or higher on at least two pairs of nip rolls and stretched in the lengthwise direction with setting the carrying rate of the nip roll on the exit side faster than that of the nip roll on the entrance side.
Japanese Patent Application Laid-Open No. 2002-311240 describes a method of longitudinal stretching of cellulose ester. In Japanese Patent Application Laid-Open No. 2002-311240, angle irregularities in the slow axis are improved by performing longitudinal stretching in a direction opposite to the film casting direction. Japanese Patent Application Laid-Open No. 2003-315551 describes a method of stretching with nip rolls disposed in a stretching zone at a small span of a length/width ratio (L/W) of 0.3 to 2. In Japanese Patent Application Laid-Open No. 2003-315551, orientation in the thickness direction (Rth) can be improved. The length/width ratio described herein means a value obtained by dividing the distance (L) between the nip rolls used for stretching by the width (W) of a cellulose acylate film to be stretched.